Gamma-ray spectrometry

ABSTRACT

Different geometries of scintillation spectrometer are disclosed which provide improved resolution over prior art scintillation spectrometers. By ensuring that photons generated by scintillation events occurring in different locations within the scintillation material generate similar light profiles on the photo-detector, the output signal is made less sensitive to the initial interaction site. This can be achieved in a number of ways, such as: by limiting the exit window of the scintillation crystal to a smaller detector, by introducing an optical spacer ( 94 ) between the scintillation crystal and detector ( 99 ), and/or by making the crystal longer than necessary to stop the gamma rays. A principal advantage of these new geometries is that deconvolution of the raw-data is more effective, thus improving resolution.

BACKGROUND ART

[0001] The invention relates to gamma-ray spectrometers, to systemsemploying gamma-ray spectrometers and to methods of gamma-rayspectroscopy.

[0002] Gamma-ray spectrometers are standard instruments used in a widevariety of scientific and industrial applications. Gamma-rayspectrometers are designed to absorb the energy of incident gamma raysand to convert the photon energy into an electronic signal proportionalto the energy deposited in the detector. These instruments are used toquantify both the energy of the gamma rays produced by a source andtheir relative intensities. This information enables the user todetermine the particular radioisotopes that are present and, forexample, their relative concentrations. For many applications, it isimportant to be able to resolve gamma ray line-features even when theyare grouped closely together. The other main performance characteristicis the stopping power of the spectrometer or its ability to absorb thegamma ray photons efficiently.

[0003] Two main types of gamma ray spectrometer are currently in use,hyper-pure germanium crystal spectrometers and scintillationspectrometers, each of which is now briefly described.

[0004]FIG. 1 of the accompanying drawings shows schematically a priorart hyper-pure germanium crystal (HPGe) spectrometer apparatus. Theapparatus comprises a detector 1 housed in a detector capsule 2 attachedto an arm 3 connected to a downwardly depending stick that is placed ina liquid nitrogen container 4. Germanium is a semiconductor having arelatively low band-gap and so has the advantage of generating oneelectron-hole pair, on average, for every 2.96 eV deposited in thecrystal. This implies that for a full energy deposit of 1MeV, the totalnumber of charge carriers produced is of the order of 350,000. Thestatistical variance in such a signal is very small and so that thisparticular contribution to the achievable energy-resolution is typicallyonly ˜0.5% FWHM.

[0005] One of the disadvantages of the HPGe detector is that it can onlyfunction as a spectrometer if cooled to liquid-nitrogen temperatures,otherwise electrons can be thermally excited into the conduction-bandand so generate a high level of noise. This means that an HPGe detectoris neither compact nor rugged. The second disadvantage is that in orderto provide a stopping power equivalent to a commonly available size ofscintillation spectrometer, the germanium crystal becomes very expensiveto fabricate. Nevertheless, such detectors are very widely used becauseof their unmatched spectral resolution.

[0006]FIG. 2 shows an example of a spectrum acquired using a HPGespectrometer (lower trace), in which energy E in keV is plotted againstnumber of counts C.

[0007]FIG. 3 shows an example of a scintillation spectrometer. Thedetector comprises a scintillation crystal 31 which scintillates when agamma-ray is absorbed within it. The scintillation crystal is fabricatedfrom a material having a high effective atomic number. The scintillationcrystal is packed into a hermetically sealed body 33 with Al₂O₃ whichacts as a reflective material and receives signal through a glassentrance window 34 situated on the upper end of the package. Thescintillation crystal is arranged above a photo-multiplier tube 32 whichis encased in a magnetic shield 35. As an alternative to aphoto-multiplier tube, a PIN diode can be used. PIN diodes areexperimentally advantageous in terms of size and ruggedness, but havenot been widely used except in small scintillation counters in which theuse of bulkier photomultiplier is inappropriate. This is because PINdiodes have relatively small sensitive detection areas and relativelyhigh noise levels. A common crystal material used for the scintillationcrystal is NaI(Tl), i.e. sodium iodide doped with a trace of thallium.In this material, an energy-loss of 1 MeV will generate 38,000 photonsin a narrow wavelength band centred on 415 nm. The sealed body 33 isneeded, since NaI(Tl) material is very hygroscopic. In conventionalscintillation spectrometers, these crystals are machined to form acylinder having a diameter, for example 78 mm (3 inches) to match thatof the photo-multiplier 32 as shown in the figure. The total dimensionsof the detector package in the specific example are 82 mm maximumdiameter and 146 mm height. The quantum-efficiency of photo-multipliersat the wavelength of interest, is typically 25% so the number ofcharge-carriers detected from a 1 MeV energy deposit is ˜8,000. Thisassumes a light-collection efficiency of 85% in transferring thescintillation light to the photo-cathode. The achievablespectral-resolution is clearly then much poorer than that of the HPGesensor since the statistical contribution to the resolution is worsethan 2.75% at 1 MeV.

[0008]FIG. 2 (upper trace) shows a typical energy-loss spectrum recordedusing an industry-standard 78 mm (3 inch) NaI(Tl) spectrometer whenilluminated by a Co-60 source. The contrast in performance with the HPGedetector (lower trace) is very clear.

[0009] The energy-resolution of a scintillation spectrometer issignificantly worse than that predicted by photon-statistics alone. Theadditional degrading effects are a consequence of several factors. Afirst effect is the variance in the scintillation efficiency of thecrystal itself, which is energy dependent and cannot be corrected forsimply. A second effect is the non-uniformity of the response of thephoto-cathode. A third effect is the variance in the light-collectionefficiency of the crystal and photo-multiplier assembly for events thatoccur in different locations within the detector crystal.

[0010] For any given field of use, the performance limitations ofscintillation spectrometers need to be compared with the experimentallimitations of HPGe spectrometers. Clearly, a gamma-ray spectrometercombining the advantages of both types of prior art device would behighly desirable. For example, if one could improve theenergy-resolution of scintillation spectrometers, there would be manyapplications of these devices for which there would otherwise be noother alternative but to use the more expensive and fragile HPGespectrometers.

[0011] One route already successfully exploited to improve theenergy-resolution of scintillation spectrometers is to apply spectraldeconvolution to the raw energy-loss data collected by the spectrometer.Deconvolution is a well known technique used in spectroscopy and otherdiverse fields, in which a raw data spectrum obtained with a detectionsystem is deconvolved with a response function representing the responseof the detection system to known input signals.

[0012] Since deconvolution is based on computing the incident spectrumfrom the energy-loss data and the detector energy-response function, itssuccess is dependent on accurately defining the detector energy-responsefunction, which is not a trivial problem.

[0013] Generally, the observed spectrum O(E) can be represented by theintegral: $\begin{matrix}{{O(E)} = {\int_{0}^{\infty}{{R\left( {E,E_{0}} \right)} \cdot {I\left( E_{0} \right)} \cdot \quad {E_{0}}}}} & (1)\end{matrix}$

[0014] where the I(E) is the incident spectrum, and R(E, E₀) is thedetector response function. This equation can be discretised as:$\begin{matrix}{\begin{bmatrix}O_{1} \\. \\. \\O_{m}\end{bmatrix} = {{\begin{bmatrix}{R_{11}R_{12}} & \ldots & R_{1n} \\{.\quad} & \ldots & . \\{.\quad} & \ldots & . \\{R_{m1}R_{m2}} & \ldots & R_{mn}\end{bmatrix} \times \begin{bmatrix}I_{1} \\. \\. \\I_{n}\end{bmatrix}} + \begin{bmatrix}ɛ_{1} \\. \\. \\ɛ_{m}\end{bmatrix}}} & (2)\end{matrix}$

[0015] Here the term ε_(i)(i=l, . . . ,m) is the noise contribution andR_(ij) is the probability that an incident gamma-ray, having an energyfalling into bin i, will be detected in bin j. An important task hasbeen to identify the most appropriate method to use in order to predictthe incident spectra, given the observed energy-loss spectra and theerrors on those spectra.

[0016] The response matrix R for a standard 78 mm (3 inch) NaI(Tl)detector can be predicted, in part, by computing how the gamma rayphotons interact in the particular scintillation material and for theparticular dimensions selected for that spectrometer. This may beaccomplished using, for example, the GEANT code developed by CERN.

[0017] This response of such a detector was calculated at 3 keV energyintervals over the range from zero to 3072 keV assuming that thegamma-ray source was located at a point 10 cm on-axis, above the top ofthe detector. An extra programme was then used to represent thebroadening of the spectrum expected as a consequence of the opticalphoton-statistics, based on the known light-yield of the materialchosen. A curve may then be fitted to the experimental data in order torepresent the way that the energy-resolution varies as a function of theincident photon energy. The effectiveness of the various mathematicalmethods that are available for use in solving this inverse problem havebeen reviewed in reference [1].

[0018]FIG. 4A shows, as an example, a raw energy-loss spectrum collectedwith a standard 78 mm (3 inch) NaI(Tl) detector illuminated by a Ra-226source, the graph plotting counts C against energy E in keV. FIG. 4B isthe equivalent graph of the deconvolved result. Not only has thespectral-resolution been markedly improved by the deconvolution, but theaccurate repositioning of the Compton-scattered events back into thefull-energy peak has resulted in an improvement of the detectorsensitivity by a factor of about four at 1MeV.

[0019] Although spectral deconvolution techniques are clearly verysuccessful in improving the results taken with scintillation detectors,the achievable improvement is limited and the quality of data taken withHPGe detectors is still far superior.

SUMMARY OF THE INVENTION

[0020] The invention has resulted from taking a holistic design view ofthe data collection (by the spectrometer) and its subsequent processing(by deconvolution). The inventors have systematically analysed theeffect that the nature of the spectrometer has on the deconvolution ofthe raw energy-loss spectrum. Through this analysis, the inventors haveidentified the fact that spectrometer design influences how good thedeconvolution can be. More specifically, the inventors have quantifiedthe limitation on the energy resolution achievable after deconvolutionthat is attributable to existing spectrometer designs. Moving on fromthat, the inventors have been able to design several new scintillationspectrometers which allow the deconvolution process to achieve a betterenergy resolution in the processed data than is possible withconventional scintillation spectrometer designs.

[0021] The invention can thus be viewed as being based on a newprinciple in which the design of the scintillation spectrometer is basedon optimising how effectively the raw-energy loss data can be treated bythe subsequent deconvolution. This contrasts with the conventionaldesign principle of concentrating on improving the energy resolution andthe signal strength of the raw-energy loss data collected by thescintillation spectrometer.

[0022] One recognition of the invention is that the scintillationspectrometer should be designed having regard to its response function,and the ease of modelling its response function, since it is thecharacteristics of the device's response function, and how accurately itcan be modelled, that determine the degree to which the raw energy-lossspectra can be enhanced by the deconvolution process.

[0023] The technical teaching of the invention may be expressed bystating that the device design should take a holistic view, in which thedeconvolution process is taken as an integral part of the design incombination with the physical design of the gamma-ray spectrometeritself.

[0024] The invention was arrived at from the following analysis of theresolution limits of a conventional 78 mm (3 inch) NaI(Tl) scintillationdetector. The results of the analysis are shown in Table 1. The overallresolution R_(overall) is shown in the right-hand column, and itscomponents, as determined by the modelling process, in the three middlecolumns. Each row of the table shows the results for different gamma-rayenergies. The components are the calculated statistical noiseR_(statistic), the calculated intrinsic noise of the scintillationmaterial R_(intrinsic) and a residual component R_(residual) obtainedfrom subtracting the statistical and intrinsic contributions from thecalculated overall resolution value R_(overall). TABLE 1 Contributionsto the Energy-resolution of the 78 mm (3 inch) NaI Detector EnergyR_(statistic) ¹ R_(residual)(R_(transfer))² R_(intrinsic) R_(overall)  80 keV (¹³³Ba)  10% 5.5% 3.7%  12%   121 keV (¹⁵²Eu) 7.7% — 4.3%˜9.5%     244 keV (¹⁵²Eu) 5.5% — 4.9% ˜8.9%     356 keV (¹³³Ba) 4.8%6.5% 4.6% 9.3%   511 keV 4.0% 6.8% 5.6% 9.7%   662 keV 3.5% 6.9% 4.6%9.0% 1.274 keV 2.5% 6.2% 3.5% 7.5%  2.22 MeV 1.9% 5.4% 3.0% 6.5%    4MeV 1.4%   ˜5%³ 1.9% 5.5%    6 MeV 1.2%  ˜5% 1.5% 5.4%    8 MeV 1.0% ˜5% 1.0% 5.2%

[0025] The Gaussian contribution was calculated for a light-yield of4500 electrons/662 keV.

[0026] The three contributions to the resolution limit are now discussedin turn.

[0027] The contribution R_(intrinsic) arises from the intrinsic variancein the light output from the scintillation material. Each scintillationmaterial has its own characteristic response when irradiated byelectrons having different energies [2]. As a consequence, one maycombine the information from the GEANT modelling exercise to estimatethe spread in the light that one would expect to be generated by theelectrons produced in the crystal by the Photo-electric, Compton andPair creation presses.

[0028] The contribution R_(statistic) arises from the statistical noiseassociated with the photon creation process. For spectometers based onphoto-multipliers, the statistical noise contribution is very small andso can be neglected. However, it does become a major contribution to thespectral-resolution of scintillation spectrometers below 1 MeV when aphotodiode is used to read-out the signal.

[0029] The contribution R_(residual) is the focus of the present designconsiderations. This contribution is a catch-all contribution that wasestimated by comparing the observed spectral-resolution R measured at anumber of different energies and progressively removing the othercontributions that are easier to model. The intuition of the inventorswas that the contribution R_(residual) arose from limitations in thedetector transfer or response function as now explained, hence thealternative labelling of is term as in the table. It is known that theresponse of the sensitive area of the photo-multiplier tube, itsphoto-cathode, varies such that a gamma-ray absorption event occurringclose to the entrance window the photo-multiplier (i.e. nearest to thephoto-cathode) will generate a significantly different sigh than anidentical event occurring near the top of the scintillation crystalfarthest from the photo-cathode (see FIG. 3 for reference). Since thephoto-cathode spatial response is variable from tube to tube, thiseffect cannot be modelled satisfactorily leading to inaccuracy incalculating the response function to be used for the deconvolution

[0030] The conclusion of the inventors was that scintillation detectorshould be redesigned to reduce this variance by ensuring that, as far aspossible no matter where the primary gamma-ray absorption event occurredin the scintillation crystal, the detector response should be the same.This conclusion be expressed by two design rules:

[0031] the path between gamma-ray absorption and detector sensitive areashould be large

[0032] the detector sensitive area should have a uniform response acrossits area.

[0033] According to a first aspect of the invention there is provided agamma-ray spectrometer comprising a scintillation body for receivinggamma-rays and creating photons therefrom, and a photon detector havinga sensitive area facing the scintillation body so as to receive anddetect the photons, wherein the sensitive area of the photon detectorpresented to receive the photons is no more than 10% of the surface areaof the scintillation body.

[0034] By making the photodetector small in comparison with the size ofthe scintillation body, the path lengths between gamma-ray absorptionand photon incidence on the photodetector can be made more equal throughthe promotion of multiple scattering, for example by multiplereflections from the outer surface of the scintillation body. Theproportion of light that reaches the photodetector directly can thus bemade small, with only scintillation events occurring in a small fractionof the volume of the scintillation body in the immediate vicinity of thephotodetector resulting in direct transfer of light from thescintillation event location to the photodetector.

[0035] Reducing the relative size of the photo-detector in this way thusserves to make the light profile at the photo-detector invariant withthe position of initial gamma-ray interaction in the scintillation body.This counter-intuitive, since the usual prior at approach is to make thephotodetector sensitive area as large as possible to increase thesignal. Typically, in the prior art, the photo-detector sensitive areais made to extend across as large a proportion as possible of thescintillation body area. We advocate the opposite approach for thereasons explained above.

[0036] It will be understood that the sensitive area of thephoto-detector may be larger than the sensitive area presented toreceive the photons. This will be the case if, for example, an apertureor small exit window is placed in front of a photo-cathode of aphotomultiplier tube to ensure that photons are only incident on arestricted central area of the photo-cathode which has greater responseuniformity.

[0037] The sensitive area is preferably between 1% and 10%, morepreferably 1% and 5%, of the surface area of the scintillation body.

[0038] The scintillation body may have at least a portion of its surfacewhich is curved, for example arcuately curved. The sensitive area of thephotodetector can then be aged generally tangenitally to the curvedsurface portion. In one embodiment, the scintillation body is geniallyspherical. Deviations from a sphere could be used, including a widevariety of generally rounded or ovoid bodies or bodies with roundedsurface parts. The advantage of adhering as closely as possible to aspherical shape is that such a shape is amenable to accurate modelling,resulting in accurate determination of the response (function used inthe post-detection deconvoluation process. As described above, this willimprove the accuracy and resolution of the data.

[0039] According to a second aspect of the invention there is provided agamma-ray, spectrometer comprising a gamma-ray spectrometer comprising ascintillation body for absorbing gamma-rays at locations with thescintillation body and creating photons therefrom and a photon detectorarranged to receive and detect the photons, wherein the photon detectoris separated from the scintillation body by a light guiding spacerhaving a length between 0.3 and 10 times the width of the scintillationbody so as to spread the photons so that their intensity profile acrossthe photon detector is relatively invariant to the locations where thegamma rays are absorbed in the scintillation body.

[0040] Inclusion of a light guiding spacer serves to make the lightprofile at the photo-detector invariant with the position of initialgamma-ray interaction in the scintillation body. This thus represents analternative way of achieving the same result as achieved in the firstaspect of the invention. This is counter-intuitive since the prior artapproach would be to maximise the signal by maximising the photon fluxincident on the photodetector. The approach of arranging a relativelythick loss-inducing spacer between scintillation body and photodetectorwould therefore not be taken.

[0041] It will be understood that the first and second aspects of theinvention are compatible and can be combined in a single device. Forexample, a light-guiding sparer may be arranged between a small-areaphoto-detector and the surface of a scintillation body.

[0042] In implementations of the second aspect of the invention, thelength of the light guiding spacer may be at least 0.4 or 0.5 times thewidth of the scintillation body and the length of the light guidingspacer my be no more 1, 2, 4, 6 and 8 times the width of thescintillation body. More generally, the length of the spacer should bechosen so the light profile at the photo-detector is relativelyinvariant with the position of initial gamma-ray interaction in thescintillation body, whether the initial-ray interaction occursimmediately adjacent to the spacer or farthest away from the spacer.

[0043] The light-guiding spacer is advantageously packed in a reflectivematerial.

[0044] In the first or second aspects of the invention the scintillationbody is advantageously packed in a reflect material.

[0045] According to a third aspect of the invention fare is provided agamma-ray spectrometer comprising a scintillation body for absorbinggamma-rays of at least a first energy at locations within thescintillation body and creating photons therefrom, and a photon detectorarranged to receive and detect the photons, wherein the scintillationbody is dimensioned to have a length of at least twice the attenuationlength of gamma-rays of the at least first energy in the scintillationbody, so as to spread the photons so that their intensity profile acrossthe photon detector is relatively invariant to the locations where thegamma rays are absorbed in the scintillation body.

[0046] By making the scintillation body longer than is necessary to stopthe incident gamma-rays of into the light profile included on thephotodetector is made invariant with the position of initial gamma-rayinteraction in the scintillation body. This is counter-intuitive, sincethe usual prior art approach is to make the scintillation body no longerthan is necessary to stop all the gamma rays. In the prior art, it wouldbe viewed as undesirable to make the scintillation body longer thannecessary to absorb the gamma rays of interest, since this will inducesignal losses as a result of the “roll-off” (i.e. photon signal decaythrough escape of photons out of the scintillation body and otherprocesses). In other words, making the scintillation body longer thanthe attenuation length of the gamma rays of interest will result infewer photons reaching the photodetector. However, the teaching of thepresent invention makes it clear that it is more important to ensure thephotodetector response is the same no matter where the initial gamma rayabsorption occurs then to maximise the signal by maximising the numberof photons incident upon the photodetector.

[0047] The third aspect of the invention is particularly suited forarray spectrometer in which the scintillation body comprises an array ofpillars. The pillars of the array are preferably laterally isolated fromeach of with reflective material. The photon detector advantageouslycomprises an array of detection element, preferably matched to pillars.The array of detection elements is made of an array of discretephotodiodes, a monolithic array of photodiodes, a multi-pixel hybridphotodiode, or an electron-bombarded charged coupled detector (CCD).

[0048] For imaging applications, the spectrometer may further comprise acoded-aperture mask, collimator, pin hole or other imaging devicearranged before the scintillation body to allow for imaging using anarray of detection elements.

[0049] A further aspect of the invention relates to a method ofgamma-ray spectroscopy comprising: providing an object to be analysedbased on gamma rays, and collecting energy-loss data for the object witha gamma-ray spectrometer comprising a scintillation body for receivinggamma-rays and creating photons therefrom, and a photon detector havinga sensitive area facing the scintillation body so as to receive anddetect the photons, wherein the sensitive area of the photon detectorpresented to receive the photons is no more than 10% of the surface areaof the scintillation body.

[0050] A still further aspect of the invention relates to a method ofgamma-ray spectroscopy comprising: providing an object to be analysedbased on gamma rays, and collecting energy-loss data for the object witha gamma-ray spectrometer comprising a scintillation body for absorbinggamma-rays and creating photons therefrom and a photon detector arrangedto receive and de the photons, wherein the photon detector is separatedfrom the scintillation body by a light guiding spacer having a lengthbetween 0.3 and 10 times the width of the scintillation body so as tospread the photons so they more uniformly illuminate the photondetector.

[0051] Another aspect of the invention relates to a method of gamma-rayspectroscopy comprising: providing an object to be analysed based ongamma rays of at least a first energy, and collecting energy-loss datafor the object with a gamma-ray spectrometer comprising a scintillationbody for absorbing the gamma-rays and creating photons therefrom, and aphoton detector arranged to receive and detect the photons, wherein thescintillation body is dimensioned to have a length of at least twice theattenuation length of gamma rays of the at least first energy in thescintillation body, so as to spread the photons so that they moreuniformly illuminate the photon detector.

[0052] The collected energy-loss data can then be processed bydeconvolution using a response function computed for the gamma-rayspectrometer. The processing may automatically compensate fortemperature effects, if the energy-loss data is collected withtemperature data indicating the scintillation body temperature.

[0053] In the above aspects of the invention, the photon detector isadvantageously based on a semiconductor element, such as a PIN diode,since these can be made with a very uniform sensitivity across theirsensitive area. In specific examples, the photon detector may be a PINdiode, a drift diode, a hybrid photodiode CHPD) or an avalanchephotodiode (APD). Alternatively, the photon detector is aphoto-multiplier tube (PMT). To improve uniformity of response the PMTmay be specially selected, or only a restricted area of thephoto-cathode presented to receive photons. For example, the restrictedarea may be a central portion of the photo-cathode over which theresponse variation is relatively small.

[0054] The gamma-ray spectrometers of the above aspects of the inventionoffer the possibility of using gamma-ray spectrometry for a variety ofexisting and new applications, such as:

[0055] for determining radioactivity levels in a soil sample,

[0056] in an airborne apparatus for mapping radioactivity levels,

[0057] for determining radioactivity levels in a liquid sample, such asaqueous effluent,

[0058] in a cooling circuit of a nuclear reactor for detecting presenceof one or more fission products in the cooling circuit,

[0059] for measuring elemental composition of crushed rock byirradiating the crushed rock with neutrons and detecting gamma raysemitted as a consequence,

[0060] for verifying the elemental composition of cement before firingby irradiating the cement with neutrons and detecting gamma rays emittedas a consequence,

[0061] for measuring the calorific content of coal by irradiating thecoal with neutrons and detecting gamma rays emitted as a consequence,

[0062] for detecting radioactive material passing through a detectionarchway or baggage control apparatus, or contained in a shippingcontainer,

[0063] for detecting explosives passing through a detection archway orbaggage control apparatus, or contained in a shipping container,

[0064] for detecting narcotics passing through a detection archway orbaggage control apparatus, or contained in a shipping container,

[0065] for detecting buried landmines by illuminating the ground withneutrons and detecting gamma rays emitted as a consequence,

[0066] for detecting and imaging contraband materials using multi-energygamma-ray computed tomography (MEGA-CT),

[0067] for analysis of the U/Th ratio in rock chippings generated duringdrilling of an oil well,

[0068] for rock composition analysis by down-the-well neutron analysis,

[0069] for rock core analysis,

[0070] for radio guided surgery,

[0071] in a gamma ray imaging system for imaging radioactive tracers ina patient,

[0072] for thickness measurements with a monochromatic beam of gammarays,

[0073] for measuring the cement content of concrete from the naturalradioactivity levels in the constituents,

[0074] for measuring the water content of concrete by illuminating theconcrete with neutrons.

[0075] These applications are described in more detail further below.

BRIEF DESCRIPTION OF THE DRAWINGS

[0076] For a better understanding of the invention and to show how thesame may be carried into effect reference is now made by way of exampleto the accompanying drawings in which:

[0077]FIG. 1 is a schematic perspective view of a section of a prior arthyper-pure germanium crystal (HPGe) spectrometer system;

[0078]FIG. 2 is a graph of a typical spectrum recorded with an HPGespectrometer and a prior art NaI(Tl) scintillation spectrometer;

[0079]FIG. 3 is a schematic section view of a prior art NaI(Tl)scintillation spectrometer;

[0080]FIG. 4A is a graph of a typical spectrum recorded with a prior artNaI(Tl) scintillation spectrometer.

[0081]FIG. 4B is a graph of a deconvolved spectrum recorded with a priorart NaI(Tl) scintillation spectrometer;

[0082]FIG. 5 is a contour graph of a typical spatial response map acrossthe photo-cathode of a photo-multiplier tube;

[0083]FIG. 6 is a graph of a typical deconvolved spectrum recorded witha prior art NaI scintillation spectrometer;

[0084]FIG. 7 is a graph of a typical deconvolved spectrum recorded witha CsI scintillation spectrometer according to an example of a firstembodiment of the present invention;

[0085]FIG. 8A is a schematic section view of a scintillationspectrometer according to a first example of a first embodiment of thepresent invention;

[0086]FIG. 8B is a schematic section view of a scintillationspectrometer according to a second example of a first embodiment of thepresent invention;

[0087]FIG. 8C is a schematic section view of a scintillationspectrometer according to a third example of a first embodiment of thepresent invention;

[0088]FIG. 9 is a schematic section view of a scintillation spectrometeraccording to a prior art configuration with a number of gamma-rayinteraction sites P1, P3, P5, P7 and P8 marked;

[0089]FIG. 10A is a schematic representation of the modelled intensitydistribution at the output of the scintillation crystal shown in FIG. 9in response to a gamma-ray interaction at site P8;

[0090]FIG. 10B is a schematic representation of the modelled intensitydistribution at the output of the scintillation crystal shown in FIG. 9in response to a gamma-ray interaction at site P1;

[0091]FIG. 11 is a schematic section view of a first example of ascintillation spectrometer according to a first example of a secondembodiment of the present invention;

[0092]FIG. 12 is a schematic section view of a second example of ascintillation spectrometer according to a second embodiment of thepresent invention with a spacer of thickness m;

[0093]FIG. 13A is a schematic representation of the modelled intensitydistribution at the output of the scintillation crystal of FIG. 12 withspacer thickness m=0 in response to a gamma-ray interaction site R8;

[0094]FIG. 13B is a schematic representation of the modelled intensitydistribution at the output of the scintillation crystal of FIG. 12 withspacer thickness m=0 in response to a gamma-ray interaction site R1;

[0095]FIG. 13C is a schematic representation of the modelled intensitydistribution at the output of the scintillation crystal of FIG. 12 withfinite spacer thickness in response to a gamma-ray interaction site R8;

[0096]FIG. 13D is a schematic representation of the modelled intensitydistribution at the output of the scintillation crystal of FIG. 12 withfinite spacer thickness in response to a gamma-ray interaction site R1;

[0097]FIG. 14 is a graph showing the fraction of generated light whichis transferred to the exit window of a scintillation spectrometer of thetype shown in FIG. 12 as a function of the length of the light guide;

[0098]FIG. 15 is a graph showing the fraction of generated light whichis transferred to the exit window of a scintillation spectrometer of thetype shown in FIG. 12 for two different gamma-ray interaction sites as afunction of the internal surface reflectivity;

[0099]FIG. 16 is a schematic section view of a third example of ascintillation spectrometer according to a second embodiment of thepresent invention;

[0100]FIG. 17A is a schematic perspective view of a scintillationspectrometer according to a first example of a third embodiment of thepresent invention;

[0101]FIG. 17B is a schematic perspective view further detailing anindividual cell within the spectrometer of FIG. 17A;

[0102]FIG. 17C is a schematic perspective view of a second example of athird embodiment of the present invention;

[0103]FIG. 18A is a schematic section of a first example of a firstapplication of an embodiment of the invention;

[0104]FIG. 18B is a schematic section of a second example of a firstapplication of an embodiment of the invention;

[0105]FIG. 19 is a schematic section of a first example of a secondapplication of an embodiment of the invention;

[0106]FIG. 20 is a schematic section of a first example of a thirdapplication of an embodiment of the invention;

[0107]FIG. 21 is a schematic section of a first example of a fourthapplication of an embodiment of the invention;

[0108]FIG. 22 is a schematic section of a first example of a fifthapplication of an embodiment of the invention;

[0109]FIG. 23 is a schematic section of a first example of a sixthapplication of an embodiment of the invention;

[0110]FIG. 24 is a schematic section of a first example of a seventhapplication of an embodiment of the invention;

[0111]FIG. 25 is a schematic section of a first example of an eighthapplication of an embodiment of the invention;

[0112]FIG. 26 is a schematic section of a first example of a ninthapplication of an embodiment of the invention;

[0113]FIG. 27 is a schematic section of a first example of a tenthapplication of an embodiment of the invention;

[0114]FIG. 28 is a schematic section of a first example of an eleventhapplication of an embodiment of the invention;

[0115]FIG. 29 is a schematic section of a first example of a twelfthapplication of an embodiment of the invention;

[0116]FIG. 30 is a schematic section of a first example of a thirteenthapplication of an embodiment of the invention; and

[0117]FIG. 31 is a schematic section of a first example of a fourteenthapplication of an embodiment of the invention.

DETAILED DESCRIPTION

[0118] 1. First Embodiment

[0119] The analysis outlined above highlighted the potential benefits ofminimising the variances in the light-collection efficiency and spatialresponse of the photodetector use. Both of these effects can beminimised by using a high quality PIN photodiode in place of aphoto-multiplier. Although PIN photodiodes have a smaller sensitive areathe that of the photo-multipliers used in current spectrometer designs,they have a very uniform response.

[0120] In order to minimise the variances in collecting thescintillation light at the photo-detector, a spherical scintillationcrystal was chosen with a dimension that the area of the PIN diode wouldbe typically only 1% of the surface area of the crystal. It was believedthat, provided a high-reflectivity packing material selected, asufficiently high light-collection efficiency would, nevertheless, beachieved. Simulations and measurements suggest that the ratio of thedetector area to that of the surface area of the detector, should notexceed a limit of 5%. At this limit some degradation in performance isalready noticeable, as a result of the variance in path length betweengeneration of the scintillation light and its collection by thephoto-detector as a function of scintillation event position is reduced.

[0121] A number of optimal Monte Carlo simulations were made to modelthe propagation of the scintillation light within the crystal sphere fordifferent values of the reflection coefficient of the packing materialand for different values of the ratio of the PIN diode area to the totalsurface area of the detector. This work indicated that even when theratio of areas was as low as 1%, the light-collection efficiency washigh and vied by typically less than 1% for 95% of the volume in a 100cc detector.

[0122] Three prototype scintillation sphere detectors were constructedto validate these predictions. The volumes of these spheres variedbetween 20 cc, 100 cc and 300 cc, referred to as the S20-, S100- andS300-PIN spectrometers in the following. The three prototypes all used a10×10 mm PIN diode to detect the signal. These devices provided a goodrange of spectrometer sizes that could provide stopping powersequivalent to those provided by the standard scintillation spectrometersthat are currently on the market. By calculating the photon responseusing measured electron response it was possible to confirm that, forthe S100-PIN spectrometer, a light-collection efficiency of 43% had beenachieved, whilst for the S300-PIN device, the light-collectionefficiency of 22% had been achieved. A comparison between theobservational data and the modelled response of these spectrometers,enabled the contributions to the variance in the detected signals to beestimated. This data is summarised for one of the detectors, theS100-PIN scintillation sphere spectrometer, in Table 2.

[0123] Whilst the unprocessed spectral-resolution of this detector maybe seen to be superior to that of a 78 mm (3 inch) NaI(Tl) detectorhaving a similar stopping power, by comparing Tables 1 and 2, the realadvantage of the scintillation sphere spectrometer design became moreapparent after deconvolving the energy-loss spectra.

[0124]FIGS. 6 and 7 compare deconvolved spectra acquired using a priorart 78 mm (3 inch) NaI(Tl) scintillation spectrometer and our S100-PINspectrometer with the same gamma-ray source. The superior performance ofour S100-PIN spectrometer is self evident. TABLE 2 Energy-resolutionsfor the 100 cc CsI Detector R_(overall) R_(overall) (es- (mea- EnergyR_(noise) ¹ R_(statistic) ² R_(LCE) R_(intrinsic) timated) sured) 100keV 42% 5.3% 1.5% 4.0% 42.5% 150 keV 28% 4.3% 1.5% 3.6% 28.4% 200 keV21% 3.7% 1.5% 3.2% 21.6% 250 keV 16.8% 3.3% 1.5% 4.5% 17.8% 300 keV 14%3.0% 1.5% 5.5% 15.4% 356 keV 12% 2.8% 1.5% 5.3% 13.5% 13.4% 400 keV10.5% 2.6% 1.5% 5.2% 12.1% 450 keV 9.3% 2.5% 1.5% 4.9% 10.9% 511 keV8.2% 2.3% 1.5% 4.4%  9.7% 9.5% 662 keV 6.3% 2.0% 1.5% 3.8%  7.7% 7.7%1.274 keV 3.3% 1.4% 1.5% 2.2%  4.5% 4.7% 2.22 MeV 1.9% 1.1% 1.5% 1.45% 3.0% 2.7-3.2% 4 MeV 1.05% 0.83% 1.5% 1.2%  2.3% 6.13 MeV 0.7% 0.68%1.5% 0.97%  2.0% 2.0% 8 MeV 0.53% 0.59% 1.5% 0.76% 1.86%

[0125] From Table 2 it can be seen that for energies below ˜1MeV, theresolution of the S100-PIN device is dominated by the contribution fromelectronic noise. This suggested that alternative read-out strategiescould provide benefits.

[0126] One alternative is to use a large-area avalanche photodiodesdiode (LAAPD).

[0127] Another alternative is to use a special photo-multiplier tube(PMT) that has a more uniform photo-cathode response than a standardPMT.

[0128] Both of these alternative readout devices provide someamplification of the charge generated by the optical photons collectedby these detectors. This enables one to reduce the noise contributionthat is inevitable when using a PIN diode. New spectrometers have beenconstructed using both APDs and PMTs. In the latter case, a new PMTdesign was selected that has a hemispherical entrance window. Only alimited region of the photo-cathode was used. This was in theexpectation that the quantum efficiency would be most uniform in thatcentral region. However, alternative designs nave since been simulatedthat are based on the use of a small central area of a more conventionalplanar PMT. These seem to offer equivalent benefits.

[0129]FIG. 8A shows a scintillation sphere spectrometer according to afirst example. The spectrometer comprises a spherical scintillationcrystal 81 a that may be fabricated using one of a variety of materials,for example, CsI(Tl) CsI(Na), or BGO etc. The size of this sphere 81 amay be chosen to suit the requirements of the particular application inorder to provide an adequate stopping power to make an efficientdetector. However, for optimum performance, the surface area of thesphere 81 a should be much larger than the area of the exit ‘window’ tothe photo-detector (>20:1). The optical finish of the spherical surfaceis important, and varies according to the physical properties of thescintillation crystal. For CsI(Tl), the optimum performance has beenachieved by initially polishing the surface and then lightly abradingit. The sphere 81 a should be packed in highly reflective material 82 a,such as MgO powder or PTFE tape. Other materials can also be used. Thesphere 81 a may be locally adapted to match either a short square, orcircular extension 84 a in order to couple a PIN detector or avalanchephotodiode detector 85 a to the sphere 81 a using an appropriate opticalglue. It is preferable to directly couple the PIN diode to the biascircuit and pre-amplifier. This may be integrated within the detectorassembly. Signal and bias connectors 87 a, 86 a are used to connect thescintillation sphere spectrometer to signal processing electronics (notshown) which may be implemented using a PC, DSP or other hardware. Thecrystal assembly is housed within a light aluminium case 83 a that isboth light-tight and moisture-proof.

[0130] The advantages of the scintillation sphere spectrometer can alsobe achieved using a photo-multiplier for the read-out rather than asilicon photo-diode. In this case, it is important to maintain the ratioof crystal surface-area to photo-cathode area, as large as possible, say25:1 in the case of the S300-PMT scintillation sphere spectrometer. Thisprovides a reasonable compromise between the contributions to thespectral-resolution by the variance in the number of photoelectronscollected and the variance expected in the light-collection efficiencywhen interactions occur in different locations within the sphere.

[0131]FIG. 8B is a second example of a scintillation sphere spectrometeraccording to the first embodiment. A scintillator crystal 81 b, packingmaterial 82 b and a housing 83 b are provided, as will be understoodfrom the corresponding items discussed above in connection with FIG. 8A.A photo-multiplier assembly 85 b contains a hemispherical head 88 bwhich is partially immersed in the scintillation crystal 81 b. Powersupply and signal connectors 87 b and 87 c connect the scintillationsphere spectrometer with external electronics (not shown) for processingthe signal.

[0132]FIG. 8C is a third example of the first embodiment. A scintillatorcrystal 81 c, packing material 82 c, a housing 83 c, a coupling 84 c andconnections 87 b, 86 b are provided, as will be understood from thecorresponding items discussed above in connection with FIG. 8A. Aphoto-multiplier assembly 85 c is provided which is generally ofconventional design, but arranged so that only a central region of thelight sensitive area is exposed in order to reduce variance of thesensitivity across the exposed area.

[0133] 2. Second Embodiment

[0134] A key advantage of the scintillation sphere spectrometer designof the first embodiment is its ability to minimise the variance in thelight-collection efficiency when gamma rays interact in differentregions of the crystal. This characteristic was also aided by the use ofa photo-detector that has a very uniform spatial-response. Theexperience gained in the design, simulation and measurement of theperformance of the scintillation sphere spectrometer devices, suggestedthe exploration of a simpler design based on the use of a prismaticdetection crystal coupled to a similar, prismatic light-guide. Thismaterial will act as an efficient light-guide provided that its surfaceis highly reflective. The purpose of introducing this light guide was toensure that the profile of the light-pool intensity incident upon thephoto-multiplier was invariant with all the different possible locationsfor the interaction of gamma rays in the detection crystal. The additionof a light guide provides a design which enables one to use the fulldiameter of conventional photo-multipliers, thus giving a very simpleconstruction that is compatible with existing standard scintillationcrystals and PMTs. A number of optical simulations were carried out inorder to establish the value of this concept, as now described.

[0135] Several optical Monte Carlo simulations were made to determinethe light distribution at the exit plane of a cylindrical scintillationcrystal. The crystal is assumed to be CsI surrounded by an efficient MgOpowder reflector. From earlier work the reflectivity of this materialwas found to be as high as 99.7%. The light distribution over the exitplane of a CsI crystal was predicted for different gamma-ray interactionpositions. These simulation were then repeated following theintroduction of a light-guide between the crystal and thephoto-multiplier and the expected light output distribution and thetotal light flux that is incident on the photo-cathode of a PMT ispredicted. The diameter of the PMT is assumed to be equal to thediameter of the crystal. The effects of the length of the light-guide onthese parameters and the effect of reducing the reflectivity of thematerial surrounding the scintillation crystal and light-guide are alsodiscussed below.

[0136] The modelled light output is first determined for the crystalshown in FIG. 9. The model used consists of a cylinder of scintillatingcrystal 991, whose diameter d, was equal to its length, l. This crystalis optically coupled to a large glass window 992.

[0137] Point-sources of light were generated at different positionswithin the crystal to simulate scintillation events occurring whengamma-rays interact with the crystal at specific locations. Thepositions used are indicated in FIG. 9 and labelled P1, P3, P5, P7 andP8.

[0138] The light distribution at the output of the crystal was thenmapped. In this case, the light intensity varies by <2% with the variousposition of interaction within the crystal. However, the distribution ofthe light over the exit plane does vary significantly. This isdemonstrated in FIG. 10A and FIG. 10B, which represent the modelledintensity distribution at the exit plane for events occurring at thepositions labelled P8 and P1 in FIG. 9 respectively. The position of thecentroid of the light-pool varies by up to 5% for different interactionpositions within the crystal. It may be noted that when the interactionposition in the crystal is remote from the exit window, the light ismuch more evenly distributed across the exit plane.

[0139] The observations made above suggest that by ensuring that nogamma-ray interactions occur close to the exit plane, the distributionof light at this exit will be spread more uniformly over the exitwindow. This can be achieved by placing a light-guide made of anon-scintillating material between the exit of the crystal and thewindow of the photo-multiplier. Such an arrangement is shown in FIG. 11,the scintillating material 1111 is separated from the large glass exitwindow 1112 by an optical spacer 1113 of length m.

[0140] In this model, both the light-guide 1113 and the crystal 1111 aresurrounded by MgO powder 1114 as a high performance reflector. Polishingthe light guide 1113 so that Total Internal Reflection (TIR) becomesimportant increased the light-collection efficiency. Again,point-sources of light were generated at different positions within thecrystal to simulate scintillation events occurring when gamma-raysinteract with the crystal at these locations. The positions used areindicated in FIG. 11 and labelled Q1, Q3, Q5, Q7 and Q8. By using alight-guide 1113 with a length in which is equal to the diameter d ofthe crystal 1111, the sensitivity of the light output distribution overthe exit window to the location of interaction, was greatly reduced. Inthis case the centroid of the light pool varied by <0.5% compared to 5%value predicted for the crystal coupled to the PMT without alight-guide. The variance of the light pool about the centroid positionwas also calculated and for the crystal alone it varied by up to 20%.However, with the light guide in place, the variance changed by <5%.

[0141] The next set of simulations investigated how changing the lengthof the light-guide and the reflectivity of the surrounding surfaceaffected the light-collection efficiency. In these simulations, theaperture at the exit window was limited to the diameter of the crystalwhich was equal to that of the active area of the photo-cathode. Aschematic example of such an arrangement is shown in FIG. 12. A lightguide 1211 of length m optically couples a scintillation crystal ofdiameter d to an exit window 1212. As described above the crystal 1211and light guide 1213 are packed in MgO powder 1214. Again, point-sourcesof light were generated at different positions within the crystal tosimulate scintillation events occurring when gamma-rays interact withthe crystal at these locations. The positions used are indicated in FIG.12 and labelled R1, R3, R5, R7 and R8.

[0142] The simulations described above were repeated this time using afinite output aperture. The light output distribution at the exit windowis shown in FIG. 13A and FIG. 13B for a crystal having no light-guide.FIG. 13A shows the modelled light intensity when a gamma-ray interactsat the position R8 within the crystal as indicated in FIG. 12 and FIG.13B shows that for a gamma-ray which interacts at position R1. FIG. 13Cand FIG. 13D show the distributions corresponding to the same modelledinteraction sites but with a light guide 1213 of diameter d/2. It isclear that the light distribution at the exit window is far lesssensitive to the interaction position when a light-guide 1213 is used.

[0143] Using this model, predictions were made of both thelight-collection efficiency, the variations in the centroid of thelight-pool, and the variance of the light distribution about thiscentroid, for different light-guide lengths. The length m of thelight-guide was varied from d/2 to 8 d. It was found that with anylength of light-guide, the variance changed with interaction position by<1%. Similarly, the centroid position changed by <3% for all interactionpositions. The change in the variance in the light pool with interactionposition, decreased slowly as the light-guide length was increased. Thiswas ˜2% for a ½ length of guide to <0.5% with a 8 d length. In allcases, it is very much less than when no light-guide is used (˜21%).

[0144] However, the total light output at the exit window does changesignificantly with light guide length. FIG. 14 is a graph which plotsthe percentage fractional yield y of the total generated light whichreaches the exit window as a function of light-guide length m relativeto the crystal diameter d. The most effective design would incorporate alight-guide having a length of between d/2 and d. It will be understoodthat the scintillation crystal considered is cylindrical, thus having across-sectional diameter. Other shapes of scintillation crystal, such ashexagonal, square etc could be used. The above references to diameter ofthe scintillation crystal will therefore be understood more generally toapply to a width of a scintillation crystal, where width will usually bea dimension measured generally parallel to the surface plane of thephotodetector.

[0145] The effect of using coatings having different reflectivity wasalso studied. The crystal was coupled to a light-guide whose length wasequal to that of the crystal. FIG. 15 is a graph which shows how thelight-collection efficiency y, as defined above, decreases as thereflectivity R decreases for interactions occurring at the siteslabelled R1 and R8 in FIG. 12. Additionally, the sensitivity of thelight output to the gamma-ray interaction position also increased to 3%at 80% reflectivity.

[0146] The simulations showed that by using a light guide (>d/2),coupled to a cylindrical crystal (diameter d), the light output is farless sensitive to the position of the gamma-ray interaction than when nolight-guide is used. By using only a short light-guide, thelight-collection efficiency is only marginally affected provided thatthe crystal and light-guide are polished and packed in an efficientdiffuse reflector. In certain situations, there may be a need to mike atrade-off between the cost of producing such a reflective coating andits performance. It is therefore, predicted that the use of a shortlight-guide between any prismatic scintillation crystal and itsassociated photo-detector, will be an effective way by which to reducethe variance in the light collection efficiency in a gamma-rayspectrometer.

[0147]FIG. 16 shows a scintillation spectrometer according to the secondembodiment. The device comprises a conventional 78 mm (3 inch) diametercylindrical scintillation crystal 91 arranged with its upper end faceadjacent to a window 92 and its lower end face abutting anon-scintillating treatment spacer 94 in the form of a quartz lightguide which is in turn arranged upon a 78 mm (3 inch) diameter PMT 99engaged in a socket 95 electrically connected by connection leads to aHV power supply collector 96 and a signal output connection 96. Atemperature sensor 911 in the form of a thermistor is also provided. Thetemperature sensor 911 is arranged close to the scintillation crystal 91and connected by an electric connection to the signal output connection96 to provide a temperature signal. These components are housed in ahermetically sealed unit 93 in the form of a light-tight moisture-proofaluminium case.

[0148] The temperature signal is supplied to the signal collectionelectronics and recorded as one or more temperature data with thespectral data. During the spectrum processing it is possible tocompensate for temperature changes in the scintillation crystal. Acalibration file to relate the output signal from such a sensor to theambient temperature is included in the software provided with thespectrometer. This enables temperature changes to be compensated forduring the spectrum-deconvolution process. A similar arrangement can befor the first embodiment, or the third embodiment described furtherbelow.

[0149] In the scintillation prism spectrometer, the scintillationcrystal 91 is fabricated to form a prism having either a circular,square or hexagonal cross-section. These particular cross-sectionsenable one to construct detectors using one of the standard range ofphoto-multipliers that have entrance windows to match those shapes. Thisdesign enables one to construct large arrays of spectrometers in whichthe individual elements are closely tessellated. However, the particularcross-section chosen, is not vitally important. The length of the prismand choice of scintillation material must be chosen to suit theparticular application. For example, for use up to 3 MeV when the sourceof illumination is on-axis, the length of a 78 mm (3 inch) diameterNaI(Tl) crystal would need to be around 78 mm (3 inch). In this case,the optimum length of light guide would be between 39 and 78 mm (1.5 and3 inches). In certain circumstances in which the photo-cathodeuniformity is poorer, a longer light-guide may be used withoutsignificant loss in performance.

[0150] The scintillation crystal should be packed in a highly reflectingmaterial, such as MgO powder or PTFE. Alternatively, other materials,for example in paper or sheet form can be used. The scintillationcrystal is coupled to the full sensitive area of the photo-multipliervia the polished quartz light-guide 94 which has the same cross-sectionas the scintillation crystal 91 and PMT 99. Although total internalreflection at the surface of this component is effective for thispurpose, the highly reflecting packing material 92 extends from thescintillation crystal 91 to the PMT 99. The light-guide 94 is coupled tothe entrance window of the PMT 99 using an appropriate optical glue.

[0151] 3. Third Embodiment

[0152] The principles established during the development of both thescintillation sphere spectrometer and the scintillation prismspectrometer have also been applied to the design of newhigh-performance position-sensitive scintillation counters. These arearrays of miniature scintillation spectrometers each having across-sectional area of, for example, 3×3 mm which are viewed either bydiscrete PIN diodes or APDs, or a monolithic array of suchphoto-detectors. Alternatively, a position-sensitive photo-detectorhaving a very uniform photo-cathode response such as a hybrid-photodiode(HPD) or an electron-bombarded charged coupled device (CCD), wouldprovide a uniform spatial response.

[0153] In such an application, the variances in the light-collectionefficiency can be reduced by making the reflectivity of the crystalsurface is as high as possible. The variance can be further reduced bydesigning the scintillation crystals in such a way that their length islonger than needed to stop, i.e. absorb, the gamma rays of interest. Thescintillation crystal elements are made preferably at least twice,typically three times, longer than the attenuation-length of theincoming gamma radiation in the crystal. This design choice iscounter-intuitive. The aim is to be able to design a detection systemwhose response characteristics are consistent and can be modelledreliably. This then permits the user to apply the spectral deconvolutiontechnique outlined above in order to obtain an unprecedented improvementin the spectral-resolution.

[0154] To quantify the lengths of scintillation material needed, theattenuation length of gamma-rays of three different energies istabulated below for several different scintillation crystals.Scintillation Atten. length Atten. length Atten. length material at 60keV (mm) at 140 keV (mm) at 511 keV (mm) Csl 0.28 2.8 24.4 Nal 0.43 4.131.2 LSO 0.60 1.1 12.2 YAP 0.98 7.0 22.4

[0155] It will be understood that data for other gamma-ray energies andother scintillation materials are well known in the art, or readilyobtainable through standard experiments.

[0156] One particular application of this technology is in nuclearmedicine. In this field, the most commonly used radioisotope Tc-99,generates gamma-ray photons having an energy of 140 keV. At present, themost widely used detector technology makes use of rectangular pillars ofCsI(Tl) machined from a block of scintillation material. The gapsbetween these pillars are filled using a white epoxy material to act asa diffuse reflector for the scintillation light. This design results inthere being a wide range of signal amplitudes when the 140 keV photonsare detected by the crystal array. This is because the photons interactthroughout the entire depth of the pillar. Since the reflectivity of thewhite epoxy is relatively poor, a wide range of signal amplitudes isgenerated by incident photons that have the same energy. This naturallydegrades the spectral-resolution of this position-sensitive detector.Frequently, this effect is compounded by its use with a photo-detectorin which the photo-cathode response varies widely across its sensitivearea. We nave demonstrated that improved spectra can in fact be acquiredby:

[0157] Making the crystal pillars longer than is necessary for theefficient detection of the gamma rays.

[0158] Improving the reflectivity of the material used between thecrystal elements. The key factor is that the ‘roll-off’ in thelight-collection efficiency along the length of such a smallscintillation crystal should be large compared with the range of depthsover which the gamma-ray interactions occur.

[0159] Using a detector having a uniform response across the entiredetection plane.

[0160] A number of measurements have been made using bothcommercially-available crystal arrays and individual crystal elements.These were fabricated, finished and packed in such a way as to minimisethe roll-off in the signal generated as a function of distance of thepoint of interaction along the crystal element.

[0161] A high-performance, position-sensitive scintillation counter canbe designed and fabricated in such a way that its performance can bemodelled accurately so as to enable the energy-loss spectra to beprocessed to yield a more accurate measurement of the incident gamma-rayspectrum.

[0162]FIG. 17A shows a detection plane according to the third embodimentin which the scintillation crystal has been machined into atwo-dimensional array of pillar-shaped elements 101 a. The individualcrystal elements 101 a have a cross-section of, for example, 3×3 mm andare packed in a highly reflective material 102 a. The crystal elements101 a are made longer than necessary to stop the incident gamma rays ofinterest. More specifically, they are sufficiently long so as to providea light-attenuation length in the crystal, between the location of thegamma ray interaction and the photo-detector, that is large comparedwith the attenuation length of the incident gamma rays in the crystalelements. These crystal elements need to be manufactured and finishedcarefully in order to ensure that they behave in a consistent manner.This is important for the success of the spectral deconvoiutionsoftware, since the same model will be used for all of these individualdetection elements.

[0163] The choice of photo-detector is limited since it is importantthat the variation in the quantum efficiency across the detection planeis uniform. This is necessary to ensure that a single model can be usedto describe the energy-response function of all the elements in thearray. One possibility is an array of discrete PIN diodes or,preferably, a monolithic array of such devices 103 a.

[0164]FIG. 17B further details an individual detection cell from thearray of detection cells shown in FIG. 17A. In addition to thescintillation crystal 101 b, the packing 102 b and the detector 102 c, aschematic representation of a gamma-ray interaction 104 b is also shown.

[0165]FIG. 17C is a schematic representation of a second example of thisembodiment of the invention. The scintillator crystal array 101 c willbe understood from the above discussion. However in this exemplaryembodiment the detector 104 c is a multi-pixel hybrid photo-diode or anelectron-bombarded CCD detector that has been fabricated in a similarway using a transfer photo-cathode technology.

[0166] Whilst the very real benefits of improving thespectral-resolution and the sensitivity of scintillation spectrometershas been demonstrated through the application of careful modelling anddeconvolution techniques, the practical application of this technologywill be greatly assisted by carrying out the spectrum-processing task innear real-time, whilst the energy-loss spectrum is being accumulated. Byusing either a fast PC (>1 GHz) or a custom-designed fast DSP/FPGAprocessor, the computed gamma ray spectrum, incident on the detector,can be displayed and up-dated within an interval of less than a fewseconds. The processing time is a function of the number ofenergy-channels required in the gamma ray spectrum and, in the case ofthe spectral imager, on the number of pixels in the detector. In thelatter case, the number of energy-channels required in many medicalimaging applications is modest but the number of pixels might typicallybe ˜1000.

[0167] 4. Applications

[0168] 4.1 Environmental Radiation Measurements

[0169] 4.1.1 Field Measurements

[0170] In order to determine the level of radioactive contaminationpresent in, for example a ‘brown-field’ land-site, samples of soil mustbe collected from a large number of locations across the site for laterassay by a laboratory-based HPGe spectrometer system. This is inevitablyan expensive and slow process. We believe that in view of the highsensitivity, good energy-resolution and ruggedness of the newscintillation spectrometer designs described here, many of these taskscould be carried out more cost-effectively on-site. This may be achievedeither by mounting the spectrometer 111 a in a shielded enclosure 112 athat has a well-defined aperture to define the area of soil to besurveyed as schematically indicated in FIG. 18A. Alternatively, samplescan be placed inside a Marinelli beaker 113 b so that the sample 114 bsurrounds the spectrometer 111 b on all but one side as schematicallyindicated in FIG. 18B.

[0171] 4.1.2 Airborne Measurements

[0172] The spread of radioactive contaminants from industrial plantsconcerned with the use or reprocessing of nuclear materials, can bemapped using large-volume scintillation spectrometers carried either bya light aircraft or a helicopter. The volume of such spectrometers istypically 3000-5000cc. Several of these detectors are used to providesufficient sensitivity to map large areas of the terrain around theplant with a resolution of a few hundred metres. We propose that thesensitivity of such measurements to measure the activity of particularradioisotopes, remotely, could be increased, within the current payloadlimits, by using an array of smaller scintillation sphere spectrometers.Furthermore, the use of an array of such detectors 121 in conjunctionwith a coded-aperture mask 122, that could be located a metre or sobelow the detection plane, as indicated in FIG. 19 could also improvethe spatial resolution of the radioactivity map by providing an image ofthe ‘scene’ beneath the aircraft. The improvement in spatial-resolutionin the map of radioactivity generated by such a survey may be expectedto be more than an order of magnitude better than using existingtechniques.

[0173] 4.1.3 Effluent Monitoring Systems

[0174] Strict controls are placed on the total level of activity ofspecific radioisotopes that may be discharged into the environmentthrough the aqueous effluent from nuclear installations. FIG. 20 shows acluster of scintillation sphere spectrometer or scintillation prismspectrometer spectrometers 131 placed in line with the discharge pipe132 to measure integrated activity discharge and also to raise an alarmwhen the instantaneous activity level exceeds a pre-set threshold. Thesensitivity of the system may be adjusted to suit the specificrequirements by increasing, or decreasing, the number of spectrometersused in the monitoring system. A shield 134 is provided to reduce theeffects of other background radiation on the measurements.

[0175] 4.1.4 Leak Detection in Cooling Systems in Nuclear Power Plant

[0176] It is important to maintain a constant check on the ‘health’ ofthe fuel elements in a nuclear reactor. Excessive local heating,corrosion, or faults in the manufacture of a fuel-element, can lead toleaks in the sealed can that encloses the fuel. A spectrometer may beincluded in the cooling circuit in order to detect promptly, any sign ofthe specific fission products that have been generated within the fuelelements. Some have used HPGe spectrometers for this purpose. However,we propose that the high spectral-resolution, sensitivity and ruggednessof a scintillation spectrometer according to any one of the embodimentsof the invention would provide a more sensitive and reliable safetysystem. The system required would be very similar to that discussedabove and shown in FIG. 20.

[0177] 4.2 Minerals Analysis Using Neutron Activation Methods

[0178] 4.2.1 Real-Time Minerals Analysis

[0179]FIG. 21 is a schematic representation of an apparatus to measurethe elemental composition of crushed rock 141. This can be determined inreal-time by irradiating it as it is carried over a collimated neutronsource 142, on a conveyor belt 143. The neutrons stimulate the emissionof gamma rays that are characteristic of each element in the rock eitherby inelastic scattering, or following the absorption of thermal neutronsby the different nuclei present in the rock sample. Either a singledetector, or an array of detectors 144 placed above the conveyor belt,need to be able to stop these energetic gamma rays (˜1-10MeV),efficiently. These detectors also should have a good enoughspectral-resolution to clearly resolve the energies of the multiplegamma-ray lines that are generated by neutron activation in order toprovide an unambiguous analysis of the rock. The use of a scintillationspectrometer according to any one of the embodiments of the invention inthis application would provide a marked improvement in the quality ofthe data obtained.

[0180] 4.2.2 Control of the Elemental Composition of Cement

[0181] Cement is manufactured from a blend of limestone and clay alongwith the addition of a number of other materials in smaller quantities.Before these materials are finally milled and fired in a kiln, it isimportant to verify that they are present in the correct proportions.This may be achieved by using a similar neutron activation process tothat described above and indicated in FIG. 21 in which the conveyor beltcarries the blend of raw materials to make cement, as it is passed overthe neutron source. The spectrometer requirements are essentially thesame as for the analysis of crushed rock.

[0182] 4.2.3 Real-Time Measurement of the Calorific Content of Coal

[0183] Large-scale consumers of coal, such as electricity generators,are very interested in quantifying the calorific content of the coal asit is transferred to the boiler. This may be measured, again using aneutron-activation technique, so that only the tonnage of carbon, ratherthan the ash content, is paid for. This technique can also be used toquantify the sulphur content of the fuel. Whilst many operators demand areal-time analysis of the coal as it is transferred to the furnace,others are happy to make these measurements on samples, in batch-mode.The principle of the technique is again similar to that outlined insection above. However, it is necessary to optimise the neutronsource-strength and the sensitivity of the spectrometer array to suitthe needs of each particular installation. The same technique could alsobe applied to other hydrocarbon fuels such as shale oil or oil.

[0184] 4.3 Security Applications

[0185] 4.3.1 Detection of Contraband Radioactive Materials

[0186] It is of great importance in restricting the proliferation ofnuclear weapons, to maintain a close watch on the transfer of fissilematerials out of nuclear installations and across national frontiers.FIG. 22 shows one or more scintillation sphere spectrometer orscintillation prism spectrometer-spectrometers 151 mounted within astandard ‘metal-detection’ archway 152 such as are commonly used toclear passengers for boarding at airports. Such an arrangement would bevery effective in detecting small quantities of radioactive materialscarried by a passenger 153. A similar spectrometer system could be usedto detect such materials concealed in a passenger's baggage.

[0187] 4.3.2 Detection and Imaging of Contraband Materials Using NeutronActivation Methods

[0188] The presence of contraband materials in a passenger's luggage 162or even in a large shipping-container, could be detected by illuminatingthe object by a collimated neutron source 161 as shown in FIG. 23. Thecontraband materials may then be detected by searching the observedgamma ray spectra for the particular combinations of gamma rayline-features expected from a number of specific explosives or othercontraband materials, such as narcotics. These features would need to bepresent in the correct ratios within a particular volume within theregion illuminated by the neutron beam, in order to flag the presence ofpotential threat material. One could define the particular volumes ofinterest in such a search by using a collimated spectrometer 163.However, an alternative configuration for use in an imaging system basedon the characteristics of a coded-aperture, has been described inreference [4]. Although that proposal envisaged the use of an array ofsmall spectrometers based on the use of simple CsI(Tl)-photodiodescintillation counters, the performance, sensitivity and specificity ofthe spectral imager would be greatly improved by the use of ascintillation spectrometer according to any one of the embodiments ofthe invention.

[0189] 4.3.3 The Detection of Buried Landmines

[0190] The elements that are commonly present in explosive materials,generate high-energy gamma-rays in the range 2-11 MeV, when irradiatedby a neutron source. In particular, explosives are nitrogen-rich whereasmost soils have a low concentration of nitrogen. One might thereforeexpect that hidden mines might be detected by illuminating a region ofthe ground using a strong neutron source and using a collimatedscintillation sphere spectrometer to record the activated gamma-rayspectrum. Alternatively, the region could be imaged using acoded-aperture mask in conjunction with an array of scintillation prismspectrometers, in order to provide a better signal to noise ratio.

[0191] 4.3.4 The Detection and Imaging of Contraband Materials UsingMulti-Energy Gamma-Ray Computed Tomography (MEGA-CT)

[0192] In the system shown in FIG. 24, an object, for example apassenger's suitcase 171, is illuminated by a radioactive source 172that produces multiple gamma-ray lines, such as Ba-133. The attenuationof each photon-energy can be measured along a large number of pathsthrough the object to be imaged from the source to an array ofscintillation prism spectrometers 173. If the source and the detectorarray are rotated about the suitcase in small incremental angles, a‘density image’, in three dimensions, can be reconstructed from theattenuation data. If the detection plane consists of just a linear arrayof elements in a single plane, then the information required to generatea 3-D image must be acquired sequentially for each slice through theobject. The combination of that image and the attenuation data at eachof the photon energies available from the source, can provide a reliableindication of the composition of the material present in the selectedregion. This data may be compared with a library of the attenuation dataof materials that provide a potential threat.

[0193] 4.4 Oil-Well Logging

[0194] 4.4.1 Near-Real-Time Analysis of the U/Th Ratio in the RockChippings Generated During the Drilling of an Oil Well.

[0195] The naturally-occurring radioactivity that is present in rock,includes various contributions from radioactive products in the decaychains of both uranium and thorium. In addition, potassium also has along-lived isotope, K⁴⁰, that produces a gamma ray feature at 1460 keV.The relative ratios of these U, Th and K features, vary in differentrock strata. Consequently, the measurement of the ratio of the strengthof these features can be used to identify the particular strata reachedwhilst drilling an oil well. This is shown schematically in FIG. 25 bothin vertical (upper) and horizontal (lower) section. The rock chippings172 that are produced during the drilling process are collected andplaced in a Marinelli beaker 174 equipped with multiple detector heads171 to increase the sensitivity and speed of the measurement. The U/Th/Kratios may be determined in near real-time using such a system.

[0196] 4.4.2 Rock Composition Analysis by Down-the-Well NeutronAnalysis.

[0197]FIG. 26 schematically shows a method of analysing rock compositionwithin a well. The composition of the rock strata 191 throughout thedepth of an oil-well 195, along with the hydrocarbon or moisture contentcan be mapped. These include the use of neutron-activation to generategamma-ray line features that indicate the type of nuclei present in therock, whilst the intensity of the scattered thermal neutrons, provide ameasure of the amount of water or hydrocarbons present in the porousrock. The use of a scintillation spectrometer 193 according to any oneof the embodiments of the invention would improve the sensitivity ofsuch instruments and specificity of these measurements. In this case theneutron source 194 may either be a radioactive source or a pulsed D-Tsource that generates more energetic neutrons. Suitable shielding 192 isused to limit the field of view of the scintillation spectrometers.

[0198] 4.4.3 Rock-Core Analysis

[0199]FIG. 27 shows a method for rock-core analysis. A continuouscylindrical sample of the rock strata 203 that has been penetrated bythe drill-bit, may be extracted from the centre of the drill tool. Arecord of the progress of the well may then be recorded by measuring thechanges in the natural activity of these samples, and density, as afunction of distance along these rock cores. The use of scintillationsphere spectrometer or scintillation prism spectrometers 202 in suchinstrumentation, would improve the accuracy of both the measurement ofthe natural radioactivity and the density of the rock sample. In both ofthese measurements, the scintillation spectrometer is shielded fromother background radiation by a lead or tungsten collimator 201 as it isscanned along the length of the rock core. A dual-energy collimatedradioactive source 204 may be used in the measurement of the rockdensity variations along the sample.

[0200] 4.5 Medical Applications

[0201] 4.5.1 Gamma-Ray Probes for Radio-Guided Surgery

[0202] Small, collimated gamma ray spectrometers, constructed usingeither scintillation or room-temperature semiconductor materials, areused to guide surgeons to the location of a radioactive tracer materialin a patients body. Such materials are injected to indicate theboundaries of a carcinoma to ensure that all of the affected tissue hasbeen removed during an operation. There are also systems that are usedto delineate the lymphatic system related to a particular tumour. Forexample, in the case of a breast tumour it is important to locate whichof the nodes in the related lymphatic system, is the one most likely tobe involved in the spread of the disease. These ‘sentinel’ lymph nodesmay be identified by scanning the detector over the region likely to beinvolved, in order to locate the ‘hot-spot’ associated with theradioactive material trapped by the node. The sensitivity of suchmeasurements depends both on the detection efficiency of the gamma rayprobe and also on the spectral-resolution that. It is able to provide.The spectral resolution is particularly important when attempting tolocate a lymph node close to the injection site for the radioactivetracer. In such case, the high background that arises from theCompton-scattering of that radiation is very high so the probe must behighly selective in detecting the un-scattered radiation. FIG. 28 showsa small scintillation spectrometer according to any one of theembodiments of the invention. The detector 212 is coupled to a miniaturephoto-multiplier 213 and may be surrounded by either a lead or tungstencollimator 211, to provide high sensitivity, good spectral-resolutionand good spatial-resolution.

[0203] 4.5.2 Gamma-Ray Imaging Systems

[0204] An array of small scintillation crystal elements, constructedaccording to the third embodiment can be used in a wide range ofapplications in nuclear medicine, radiology and environmental radiationimaging. Such a detection plane may be used in conjunction with avariety of image-formation processes to provide a spectral imager havinga uniquely good spectral resolution. For example, when used in agamma-camera for nuclear medicine applications, the improvedspectral-resolution should reduce the background from scatteredradiation very significantly. This would permit either a lower dose ofactivity to be administered to the patient or to enable multi-energystudies to be made using two or three isotopes simultaneously. Aschematic example of such an arrangement is shown in both vertical(upper) and horizontal (lower) section in FIG. 29. The crystal array 222may be viewed either by an array of photo-diodes, or otherphoto-detector having a uniform response 223. The signals from to eachpixel are analysed and deconvolved individually, by a fast processor 224using the techniques outlined above to provide the improve spectralresolution. Different image formation techniques based for example onthe use of a collimator or a coded-aperture mask 221 are possible.

[0205] 4.6 Non-Destructive Testing

[0206] 4.6.1 Thickness Measurements

[0207] The thickness of, for example, a steel plate 231 or pipe can, inprinciple, be measured by using a collimated beam of monochromatic gammarays 232 to illuminate the object as indicated in FIG. 30. If asimilarly collimated detector 233, is used to observe the region thoughwhich the gamma ray beam passes, the scattered gamma ray spectrum thatis incident on this detector, can be processed to provide a measure ofthe thickness of the target material. The precision of this measurementdepends on the energy resolution and sensitivity of the scintillationspectrometer.

[0208] 4.6.2 Real-Time Measurement of the Cement Content of Concrete

[0209] Every mineral material has its own characteristic naturalradioactivity. This long-lived radiation originates as a consequence ofthe geological processes that led to the formation of the various rockand clay strata since the elements was formed. This naturalradioactivity is at a low level and it is necessary to use aspectrometer that has both a good energy-resolution and a highsensitivity, in order to measure the intensity and nature of theradiation generated, in a time that is practical for use on aconstruction site. If one can measure separately, the specificactivities of the feed-stock used for the cement, sand and aggregate, itis then possible to determine the cement-content of the resulting wetconcrete mix in near real-time by placing a sample of the mix in aspecial Marinelli beaker. Whilst it has been shown that such ameasurement could in principle be made using an HPGe detector [3], themeasurement-time used was of the order of 12 hours. However, by using asmall open-spaced array of several scintillation sphere spectrometers orscintillation prism spectrometers 245 as indicated both in horizontal(upper) and vertical (lower) section in FIG. 31, one could provide anaccurate measurement of the cement content of a wet concrete mix 243held in a Marinelli beaker 244 on a time-scale of the order of tenminutes. The water content of this mix could also be estimated from theintensity of the deuterium gamma ray line-feature generated by theabsorption of thermal neutrons by hydrogen. This measurement wouldrequire the positioning of a collimated neutron-emitting radioactivesource 241, such as Cf-252, above the sample of concrete.

5. REFERENCES

[0210] 1. L. J. Meng, D. Ramsden, Inter-comparison of three algorithmfor gamma-ray spectrum deconvolution IEEE Transactions on NuclearScience, volume 47, pages 1011-1117

[0211] 2. B D Rooney and J D Valentine, Scintillation light-yieldnon-proprtionality: Calculating photon response using measured electronresponse. IEEE Trans Nuclear Science, volume 44, pp 190 (1997)

[0212] 3. C G Rowbottom, W. B. Gilboy and D. J. Hannant, “Determinationof cement content of cement blends using Gamma-ray spectroscopy”, Cementand Concrete Research, Vol. 27, No. 11, pages 1681-1690 (1997)

[0213] 4. R J Evans, I D Jupp, F Lei, D Ramsden, Design of a large-areaCsI(Tl) photo-diode array for explosives detection by neutron-activationgamma ray spectroscopy Nuclear Instr and Methods A, volume 422 (1999),pp 900

[0214] The contents of these references are incorporated herein byreference in their entirety.

1. A gamma-ray spectrometer comprising a scintillation body forreceiving gamma-rays and creating photons therefrom, and a photondetector having a sensitive area facing the scintillation body so as toreceive and detect the photons, wherein the sensitive area of the photondetector presented to receive the photons is no more than 10% of thesurface area of the scintillation body.
 2. A gamma-ray spectrometeraccording to claim 1, wherein the sensitive area is between 1% and 10%,more preferably 1% and 5%, of the surface area of the scintillationbody.
 3. A gamma-ray spectrometer according to claim 1 or 2, wherein thescintillation body has at least a portion of its surface which iscurved, and wherein the sensitive area is arranged tangentially to thecurved surface portion.
 4. A gamma-ray spectrometer according to claim3, wherein the scintillation body is generally spherical.
 5. A gamma-rayspectrometer according to any one of the preceding claims, wherein thephoton detector is separated from the scintillation body by a lightguiding spacer having a length between 0.3 and 10 times the width of thescintillation body.
 6. A gamma-ray spectrometer comprising ascintillation body for absorbing gamma rays at locations within thescintillation body and creating photons therefrom, and a photon detectorarranged to receive and detect the photons, wherein the photon detectoris separated from the scintillation body by a light guiding spacerhaving a length between 0.3 and 10 times the width of the scintillationbody so as to spread the photons so that their intensity profile acrossthe photon detector is relatively invariant to the locations where thegamma rays are absorbed in the scintillation body.
 7. A gamma-rayspectrometer according to claim 6, wherein the length of the lightguiding spacer is at least 0.4 or 0.5 times the width of thescintillation body.
 8. A gamma-ray spectrometer according to claim 6 or7, wherein the length of the light guiding spacer is no more than 1, 2,4, 6 and 8 times the width of the scintillation body.
 9. A gamma-rayspectrometer according to claim 6, 7 or 8, wherein the light-guidingspacer is packed in a reflective material.
 10. A gamma-ray spectrometeraccording to any one of the preceding claims, wherein the photondetector is based on a semiconductor element.
 11. A gamma-rayspectrometer according to any one of claims 1 to 9, wherein the photondetector is a PIN diode, a drift diode, a hybrid photodiode (HED) or anavalanche photodiode (APD).
 12. A gamma-ray spectrometer according toany one of claims 1 to 9, wherein the photon detector is aphoto-multiplier tube (PMT).
 13. A gamma-ray spectrometer according toany one of the preceding claims, wherein the scintillation body ispacked in a reflective material.
 14. A gamma-ray spectrometer comprisinga scintillation body for absorbing gamma-rays of at least a first energyat locations within the scintillation body and creating photonstherefrom, and a photon detector arranged to receive and detect thephotons, wherein the scintillation body is dimensioned to have a lengthof at least twice the attenuation length of gamma rays of the at leastfirst energy in the scintillation body, so as to spread the photons sothat their intensity profile across the photon detector is relativelyinvariant to the locations where the gamma rays are absorbed in thescintillation body, wherein the scintillation body is made of CrI andhas a length of at least 0.56, 5.6 or 48.8 mm, the scintillation body ismade of NaI and has a length of at least 0.86, 8.2 or 62.4 mm,the-scintillation body is made of LSO and has a length of at least 1.2,2,2 or 24.4 mm, or the scintillation body is made of YAP and has alength of at least 1.96, 14.0 or 44.8 mm.
 15. A gamma-ray spectrometeraccording to claim 14, wherein the scintillation body comprises an arrayof pillars.
 16. A gamma-ray spectrometer according to claim 15, whereinthe pillars are lately isolated from each other with reflectivematerial.
 17. A gamma-ray spectrometer according to any one of claims 14to 16, wherein the photon detector comprises an array of detectionelements.
 18. A gamma-ray spectrometer according to claim 17, whereinthe array of detection elements is made of an array of discretephotodiodes, a monolithic array of photodiodes, a multi-pixel hybridphotodiode, or an electron-bombarded charged coupled detector (CCD). 19.A gamma-ray spectrometer according to claim 17 or 18, fur comprising acoded-aperture mask, collimator or pin hole arranged before thescintillation body to allow for imaging across the array of detectionelements.
 20. A gamma-ray spectrometer according to any one of thepreceding claims, further comprising a temperature sensor for measuringtemperature of the scintillation body.
 21. A method of gamma-rayspectroscopy comprising: providing an object to be analysed based ongamma rays, and collecting energy-loss data for the object with agamma-ray spectrometer comprising a scintillation body for receivinggamma-rays and creating photons therefrom, and a photon detector havinga sensitive area facing the scintillation body so as to receive anddetect the photons, wherein the sensitive area of the photon detectorpresented to receive the photons is no more than 10% of the surface areaof the scintillation body.
 22. A method of gamma-ray spectroscopycomprising: providing an object to be analysed based on gamma rays, andcollecting energy-loss data for the object with a gamma-ray spectrometercomprising a scintillation body for absorbing gamma-rays and creatingphotons therefrom, and a photon detector arranged to receive and detectthe photons, wherein the photon detector is separated from thescintillation body by a light guiding spacer having a length between 0.3and 10 times the width of the scintillation body so as to spread thephotons so that they more uniformly illuminate the photon detector. 23.A method of gamma-ray spectroscopy comprising: providing an object to beanalysed based on gamma rays of at least a first energy, and collectingenergy-loss data for the object with a gamma-ray spectrometer comprisinga scintillation body for absorbing the gamma-rays and creating photonstherefrom, and a photon detector arranged to receive and detect thephotons, wherein the scintillation body is dimensioned to have a lengthof at least twice the attenuation length of gamma rays of the at leastfirst energy in the scintillation body, so as to spread the photons sothat they more uniformly illuminate the photon detector.
 24. A methodaccording to claim 21, 22 or 23, further comprising: processing theenergy-loss data by deconvolution using a response function computed forthe gamma-ray spectrometer.
 25. A method according to claim 24, whereinthe energy loss-data is collected with temperature data indicating thetemperature of the scintillation body, and the processing compensatesthe energy-loss data in respect of temperature using the temperaturedata.
 26. Use of a gamma-ray spectrometer according to any one of claims1 to 19: for determining radioactivity levels in a soil sample, in anairborne apparatus for mapping radioactivity levels, for determiningradioactivity levels in a liquid sample, such as aqueous effluent, in acooling circuit of a nuclear reactor for detecting presence of one ormore fission products in the cooling circuit, for measuring elementalcomposition of crushed rock by irradiating the crushed rock withneutrons and detecting gamma rays emitted as a consequence, forverifying the elemental composition of cement before firing byirradiating the cement with neutrons and detecting gamma rays emitted asa consequence, for measuring the calorific content of coal byirradiating the coal with neutrons and detecting gamma rays emitted as aconsequence, for detecting radioactive material passing through adetection archway or baggage control apparatus, or contained in ashipping container, for detecting explosives passing through a detectionarchway or baggage control apparatus, or contained in a shippingcontainer, for detecting narcotics passing through a detection archwayor baggage control apparatus, or contained in a shipping container, fordetecting buried landmines by illuminating the ground with neutrons anddetecting gamma rays emitted as a consequence, for detecting and imagingcontraband materials using multi-energy gamma-ray computed tomography(MEGA-CT), for analysis of the U/Ti ratio in rock chippings generatedduring drilling of an oil well, for rock composition analysis bydown-the-well neutron analysis, for rock core analysis, for radio guidedsurgery, in a gamma ray imaging system for imaging radioactive tracersin a patient, for thickness measurements with a monochromatic beam ofgamma rays, for measuring the cement content of concrete from thenatural radioactivity levels in the constituent for measuring the watercontent of concrete by illuminating the concrete with neutrons formeasuring the water content of concrete by illuminating the concretewith neutrons.